Why do you feel groggy after oversleeping?

If you sleep too much, you don't feel refreshed; instead you feel sluggish, groggy, and generally disposed to more sleep. What is the scientific reason behind this?

— Mimi Thomas, via AOL

Cecil replies:

Glad you want the scientific reason, Mimi, as opposed to the tawdry rumors. Unfortunately, in this case there's not much difference, as we shall see.

More has been written about oversleeping in the medical journals than you might think. In 1969 sleep researchers John Taub and Ralph Berger gave the phenomenon (or one aspect of the phenomenon) a name: the Rip Van Winkle Effect, the experience of feeling bad after extended sleep (more than 10 hours). Various studies by the above and other parties have established that:

1. Some people feel terrible after too much sleep, and their performance deteriorates. On the other hand,

2. Some people feel great. Always the way.

3. Experiments seemed to bear out the initial hypothesis that either too much or too little sleep would cause your mental state to crumble. But by the early 80s, a few investigators had concluded that:

4. Oversleeping made people feel terrible if they'd previously had sufficient sleep, but it made them feel great if they'd previously been sleep deprived. You have to wonder why it took 12 years to come up with this brainstorm. One can only guess that sleep researchers take a lot of naps.

5. Further research in 1985 found that "with or without a prior sleep debt, the subjects' alertness was either unchanged or improved after acute oversleeping. Furthermore, actually sleeping more proved to be better for subjectively reported mood and objectively measured alertness than simply lying in bed awake for the extra hours." In other words, the Rip Van Winkle Effect is a crock, and you don't really feel bad after oversleeping. You just think you do.

Attempting to salvage something from this fiasco, the sleep research community now offers such conjectures as the following: "People generally expect to feel better after getting a long night of sleep; their expectations may predict greater improvement than they actually obtain, in which case they feel worse" (Encyclopedia of Sleep and Dreaming). Whoever wrote that was clearly feeling a little groggy. Probably got too much sleep.